This invention relates generally to a method and apparatus for measuring the distance between upper and lower surfaces of a transparent coating upon a planar surface, and more particularly to a scanning angular fringe spectrometer for measuring the absolute thickness of a dust defocus layer upon a spinning optical disk.
The recording layer of optical disks are frequently protected from scratches and dust particles by a transparent overcoat layer, sometimes known as a dust defocus layer, for keeping such effects out of focus. Since typical servo control objective lenses which are used to read optical disks generally require negative spherical aberration to counter the effect of focusing through air followed by a refractive layer, the thickness range of the dust defocus layer within which the lens can properly focus is severely limited. It has, therefore, been desirable in the past to monitor the thickness of such dust defocus layers for quality control in the production of optical disks.
Various thickness monitoring approaches have been considered in the past. For example, Southwell discloses in U.S. Pat. No. 3,994,599 a method of measuring the wall thickness of a tubular glass article by comparing the spacing between an interference fringe pattern formed upon a screen with a predetermined standard, a reject signal being created when the fringe spacing deviates from the predetermined standard by a predetermined amount. Such a method, however, is dependent upon the establishment of a standard for comparison, as well as the requirement for a tubular glass article in order to cause the angular fringes. As a result, the method and apparatus disclosed by Southwell is inappropriate for determining the absolute thickness of a dust defocus layer upon a planar optical disk.
A related approach, disclosed by Gaston et al in U.S. Pat. No. 4,293,224, utilizes the cyclic patterns of intensity change to wavelengths which are compared to identify the absolute thickness of a transparent film during microelectronic fabrication processes. A reflected white light, modified by optical interference in the transparent dielectric film, is monitored by photodetectors at two distinct wavelengths selected so that some particular coincidence of extrema in the two signals occurs at a film thickness less than the expected minimum initial thickness, and does not occur at any greater thickness up to and including the expected maximum. While the apparatus disclosed by Gaston et al is suitably inexpensive, the range of thicknesses which may be monitored is apparently limited to path length differences of less than 20 micrometers. Furthermore, there are restrictive choices in both the selection of wavelengths employed, and the positioning of the fringe patterns. Similar approaches use a spectrophotometer which scans wavelengths for a point on the coating. By monitoring the shift in fringe order number, .DELTA.m, thickness of the coating can be determined from the equation ##EQU1## for normal incidence where n is the index of refraction and .lambda..sub.i are the end-point wavelengths. One drawback to such methods, however, is the requirement for temporal fringe counting. It is, therefore, desirable to employ a method for measuring the absolute thickness of a dust defocus layer upon an optical disk wherein the method is not time dependent.
A further drawback to each of the above described apparatuses is their relative complexity of operation. In manufacturing processes such as the fabrication of optical disks, it is desirable to minimize both the complexity of design and cost of construction. Accordingly, simple apparatuses such as that disclosed by Hershel et al in U.S. Pat. No. 4,453,828 are preferably used to accurately measure the thicknesses of thin, optical membranes. By directing a light beam onto a representative thin, optical membrane, and varying the angle of incidence of the light beam upon the membrane, Hershel et al detects the angles of incidence of the light beam on the membrane, including the null angles where the membrane reflects substantially none of the incident light, and calculates the optical thickness of the membrane from one or more such null angles. While the method and apparatus for measuring the thickness of thin, optical membranes disclosed by Hershel et al may be appropriate for membranes having a nominal thickness in the range of about 0.5 to 10 micrometers, its application of measuring one or more fringes by sensed nulls is inappropriate for accuracy considerations because the sinusoidal-like fringes are better specified at the steep slope regions, or midway between the nulls and the peaks. As is well known, such nulls and peaks are actually zones of low sensitivity "peaks." Moreover, such a method is incapable of dynamically scanning the membrane to determine its thickness, and is therefore inappropriate for use in monitoring the absolute thickness of the dust focus layer upon a spinning optical disk having a nominal thickness of approximately 200 micrometers.